The disclosed system relates to metal oxide varistors (MOVs).
Manufacture of a MOV typically involves sintering metal oxide ceramic powder to provide a disc (may also be square or other shapes) body, firing electrodes onto the disc body, attaching leads typically by means of soldering and encapsulating. However it is well known that the choice of encapsulant is critical to ensure good electrical stability over time. It has been found that most standard encapsulation materials will lead to increased leakage and or a drop in the nominal voltage when subject to a biased elevated temperature test, typically called an Accelerated Life Test, e.g. 125° C., at rated bias voltage for 1000 hours.
A conventional approach to addressing this problem is to develop or select a specific material which does not exhibit this problem. Another approach is to apply a passivation material to the exposed surface of the MOV unit to prevent the surface interaction. Developing a custom ensapsulant material is time-consuming and typically results in a non-standard material with associated impact on unit cost. For high volume products, a custom enscapsulant is a suitable approach. However this may not suit low volume products. Therefore, many suppliers take the approach of applying a passivation material to the exposed MOV surface.
Applying a passivation material requires a separate step in the manufacturing process, and it is often difficult to achieve good uniformity of the passivation material coverage.
The disclosed system is therefore directed towards providing an improved method of preventing encapsulant/ceramic interaction from causing faults.